To reinforce PLA-based matrices, use is conventionally made of impact modifiers, which, on account of their size (greater than 80 nm) and their optical index, scatter light once introduced into the matrix, which affects the transparency thereof. A reinforcer based on poly(methyl methacrylate) would have the same drawbacks. In addition, the heat resistance and the impact strength of PLA-based matrices reinforced with poly(methyl methacrylate) often needs to be improved, for example by incorporating therein an impact modifier of the methyl methacrylate/butadiene/styrene type such as the Clearstrength® impact modifiers from Arkema.
Another solution proposed in document WO 2007/084 291 for improving the flexibility and elongation of PLA without substantially affecting its transparency consists in mixing it either with a PLA-grafted acrylic copolymer, or with a block copolymer containing PLA blocks and blocks of a (meth)acrylic homopolymer, to form (meth)acrylic copolymer microdomains dispersed in a PLA matrix, or vice versa.
There is, however, still a need for another means for reinforcing PLA-based matrices without substantially affecting their transparency. Now, such matrices are known to be poorly compatible with other polymers, insofar as they are liable to be the site of chemical degradations leading to coloration of the matrix and to a loss of molecular weight. In addition, the adherence at the interface of the two materials is not always satisfactory, which affects the physical properties of the reinforced matrix and may in particular make it brittle.
The Applicant has, to its merit, developed mixtures of PLA with block copolymers that make it possible to overcome the deficiencies of the prior art.
These copolymers make it possible to obtain an organization of the material at the nanometer scale, not via mechanical mixing but via thermodynamically-governed self-organization of the molecules. This results in very significant reinforcement of the PLA-based matrix without substantially affecting its other properties and especially its transparency and its rigidity.
It is already known from document US 2004/0 147 674 that the flexibility and impact strength of a thermo-plastic resin such as PLA may be improved by combination with a linear or star block acrylic copolymer, such as a methyl methacrylate/n-butyl acrylate/methyl methacrylate triblock copolymer. This copolymer has a polydispersity index Ip=Mw/Mn (in which Mw is its weight-average molecular mass and Mn is its number-average molecular mass) of not more than 1.8 and preferably of not more than 1.5. It may be prepared by living anionic polymerization, radical polymerization using a chain-transfer agent or living radical polymerization using, for example, a nitroxide. Atom-transfer radical polymerization, initiated with an organic halide and catalyzed with a metallic complex is, however, preferred.
Along the same lines, it has been envisaged in document WO 2007/060 930 to combine PLA with block acrylic copolymers, especially of the methyl methacrylate/butyl acrylate/methyl methacrylate type, in a PLA/copolymer weight ratio ranging from 97/3 to 40/60. These copolymers are obtained according to an atom-transfer radical or anionic polymerization process, in the presence of a transition metal and a halogenated compound used as initiator. This process gives them a polydispersity index of between 1 and 1.4.
Although the mixtures described in document WO 2007/060 930 are presented as having improved properties in terms of flexibility, moldability, hot-bonding ability, permeability to moisture, impact strength, flexural strength and resistance to elongation, without loss of transparency, these properties are obtained only for combinations that have a certain ratio of melt viscosities of the two constituents of the mixture. In particular, Table 2 of said document shows that compositions containing a weight ratio of PLA to the copolymer of 30/70 cannot afford the desired properties.